This invention relates to an electrochromic display device having a selectively colorable tungsten oxide film in contact with an acidic aqueous electrolyte. More particularly, this invention relates to improving the useful lifetime of an electrochromic device by minimizing the dissolution of the tungsten oxide film into the acidic aqueous electrolyte.
In a conventional electrochromic display device, the desired display is formed by applying an electric field to selectively color an electrochromic tungsten oxide film of empirical formula WO.sub.3. Typically, the display comprises a supporting glass plate having a transparent tin oxide electrode on one surface thereof and a transparent WO.sub.3 thin film vapor-deposited onto the tin oxide electrode. The deposited WO.sub.3 film is contacted with an acidic aqueous electrolyte, for example a 10% by volume H.sub.2 SO.sub.4 solution, which is in turn in contact with a suitable counterelectrode.
The deposited WO.sub.3 film is colorless. The display is developed by connecting the tin oxide electrode to a negative battery terminal and the counterelectrode to the positive terminal. The applied electrical field and protons from the electrolyte cause the film to change colors in accordance with the following half-reaction: EQU xH.sup.+ +xe.sup.- +WO.sub.3 (colorless).rarw..fwdarw.H.sub.x WO.sub.3 (Blue)
wherein the extent of reaction x is controlled by the current entering the film and is proportional to the depth of coloration. The blue color, designated tungsten bronze, persists after the electric field is discontinued. However, by reversing the polarity of the electrodes, the reaction is reversed, the electrochromic film is bleached, and the display is erased.
A major problem encountered with conventional electrochromic display devices is that the tungsten oxide film dissolves in the aqueous electrolyte when the device is simply sitting on the shelf. The rate of film dissolution is substantially increased by the cyclic operation of the device, that is, by the alternately coloring and bleaching of the film. Thus, the device has a short shelf lifetime and an even shorter cycle lifetime.
Since film dissolution depends upon the presence of water in the electrolyte, attempts to reduce film dissolution have included substituting, for the aqueous electrolyte, an electrolyte consisting of a suitable acid dissolved in an organic solvent, such as glycerine. The use of an organic-base electrolyte improves the device lifetime, but requires a substantially longer switching time, the time required to color or bleach the film. Thus, attempts to minimize water in the electrolyte have failed to produce a suitable electrochromic device.
Another approach for reducing film dissolution has been to saturate an aqueous electrolyte with dissolved tungsten oxide salts. In practice, the use of a saturated electrolyte has failed to significantly extend the device lifetime. It has also been proposed to place adjacent the electrochromic film a porous membrane permeated with electrolyte. For example, a sponge-like polymer matrix carrying electrolyte minimizes contact between the electrolyte and the film and thereby reduces dissolution. The use of a porous layer has not been successful in eliminating film dissolution and has the obvious disadvantage of requiring additional materials to construct the device.
Therefore, it is an object of this invention to improve the shelf and cycle lifetimes of an electrochromic display device having an electrochromic tungsten oxide film in contact with an acidic aqueous electrolyte by substantially reducing the dissolution of the film into said electrolyte, but without impairing the desired display-forming operation of the device.
It is a further object of this invention to provide, in an electrochromic display device, an electrochromic tungsten oxide film having a treated surface, which surface prevents the dissolution of the film into the electrolyte, but readily conducts protons to effect the desired coloring and bleaching reactions within the underlying film. The treated film surface of this invention is not itself electrochromic, but acts as a physical barrier to the dissolution of the underlying electrochromic film material.
It is a still further object of this invention to provide a method for treating a vapor-deposited electrochromic tungsten oxide film to produce a surface that is insoluble in an acidic aqueous solution, but which treatment does not adversely affect the ability of the film surface to readily conduct protons. When the treated film of this invention is incorporated into an electrochromic display device, the treated surface prevents the film from dissolving into the acidic aqueous electrolyte and readily conducts protons into and out of the film to effect the desired coloring and bleaching reactions. Thus, an electrochromic device having a tungsten oxide film treated in accordance with this invention evidences both a substantially extended useful lifetime and a fast switching time.